The subject matter of this application is directed to a metal-oxide-semiconductor field-effect transistor (MOSFET) sampling switch and particularly to a MOSFET sampling switch that reduces parasitic capacitance in PMOS or NMOS transistors.
MOSFETs are widely used in many type of digital and analog circuits. The two type of MOSFETS widely used in the industry are NMOSFET (n-type MOSFET, NMOS, or NFET) and PMOSFET (p-type MOSFET, PMOS, or PFET). PMOS and NMOS transistor can be included in a complementary metal-oxide-semiconductor (CMOS) which typically uses both complementary and symmetrical pairs of PMOS and NMOS transistors. CMOS device are used in many types of analog circuits because the characteristics of the circuit can be controlled by changing the size of the components in the transistors and because the transistors provide nearly ideal switching characteristics.
PMOS and NMOS transistors have been used in circuits that integrate digital and analog functions. For example, PMOS and NMOS transistors have been used in switched-capacitor circuits to perform voltage sampling of time varying voltages. The voltage sampling of the time varying voltage can be achieved by coupling a switch, implemented by at least one of PMOS and NMOS transistor, to a sampling capacitor. The input signal can be coupled to the capacitive storage element by turning “on” and “off” the switch. These switches can be turned “on” and “off” by controlling the voltage applied to the gate electrode of the PMOS or NMOS transistor. Switched-capacitor circuits can be used in gain stages, comparators, filters, digital-to-analog converters (DACs), analog-to-digital converter (ADCs), sample-and-hold amplifiers (SHAs) and in many other applications.
Advances in manufacturing techniques to produce smaller MOSFET devices have allowed MOSFETs to be used in applications needing higher processing speed, reduced power consumption, and reduced space consumption. For example, reduction in the size of MOSFETs typically decreases the supply voltage because a smaller gate drive voltage can be used to control the MOSFET. However, reducing the size of the MOSFETS does not eliminate all of the design challenges and can introduce new challenges. For example, although the “on” resistance in the transmission gate, between the source and the drain of the transistor, of the MOSFET may decrease due to decreased process geometry, the “on” resistance may still affect the operation of the transistors. Furthermore, when a voltage is applied to the gate, to turn “on” the MOSFET, the “on” resistance of the MOSFET is a nonlinear function of the signal voltage coupled by the MOSFET.
In addition, MOSFET devices have parasitic capacitances that can be formed at the borders between the different regions of the MOSFET device. For example, parasitic capacitances can be formed between the gate and a back gate, between the source and the gate, between the source and the back gate, between the drain and the gate, and between the drain and the back gate. In particular, when the MOSFET device is in the “on” state, the signal voltage coupled by the MOSFET device will see an undesired reverse depletion capacitance between the drain of the MOSFET device and a substrate on which the MOSFET is manufactured (back gate) and between the source of the MOSFET device and the substrate (back gate). These parasitic capacitances can induce frequency-dependent and voltage-dependant corruptions and introduce signals passing though the transistors which can lead to signal error in the circuits that use them.
Accordingly, the inventors have identified a need in the art to minimize the influence of parasitic capacitances on circuit performance. In particular, the inventors have identified a need in the art to minimize the influence of parasitic capacitances due to the junction capacitance between the drain and the substrate and between the source and the substrate.